The present invention relates generally to optical phased arrays and more particularly to control systems for optical phased arrays that compensate for phase changes due to external perturbations such as vibration and thermal variations as well as compensating for atmospheric aberrations external to a laser system.
Optical phased arrays are well known in the art and are used in a variety of applications for the purpose of generating an optical output having greater power than an optical input through the use of an array of emitters. More specifically, optical phased arrays have been used in laser systems to generate an output beam having high power levels in addition to high beam quality or brightness. High beam quality allows available light to be concentrated onto a small area at a large distance, which is advantageous in laser weapons systems.
The optical phased array is similar to phased array radar, wherein an array of emitters must be emitting light at precisely the same phase. However, matching the phases of the emitters at optical frequencies is more difficult than with radio frequencies, as used with phased array radar, due to the short wavelength of light. Additionally, external perturbations such as vibrations and thermal variations continuously alter the phase of each emitter. Further, atmospheric aberrations external to the laser system also change the phase of the emitters, wherein a control system may generate an arbitrary phase front to compensate for atmospheric propagation perturbations. Accordingly, a control system must be employed to realign the emitters to compensate for phase changes due to external perturbations and atmospheric aberrations.
Generally, control systems measure the phase of each emitter compared with the phase of a reference signal and feedback phase correction signals to phase modulators that realign the phase of each emitter. Such techniques are referred to as near-field techniques, wherein the phase of each emitter is measured and controlled by using an interferometer to compare the phase of each emitter to the phase of a reference signal. See, for example, U.S. Pat. No. 5,694,408 to Bott et. al., assigned to McDonnell Douglas Corporation, the disclosure of which is incorporated by reference in its entirety herein. The phase modulators may realign the phases of the emitters to match a reference signal, or alternately, the phase modulators may realign the phases of the emitters to produce an output beam having a specific phase profile or phase taper. Furthermore, microprocessors having algorithms to calculate phase measurements and corrections are also employed in the known art.
Unfortunately, the algorithms that are used in the known art to calculate phase measurements and corrections are computationally intensive and thus may not be capable of providing real-time feedback to the phase modulators if the optical phased array comprises a large number of emitters. For example, known art algorithms that use the near-field technique repeatedly sweep a reference beam by 2xcfx80 radians while measuring the output for each emitter. Generally, timing information is used from the start of the sweep to the time when the output appears to determine the phase of the emitter, and the algorithm is further sensitive to the AC amplitude of the interferometer signal and also to the DC component thereof. As a result, the algorithm is computationally intensive and consumes valuable time that is needed to continuously correct the phases of the emitters. Additionally, conventional techniques are phase-nulling, and thus the control system cannot lock to an arbitrary phase front to compensate for atmospheric propagation perturbations.
In addition to the near-field techniques, far-field techniques have also been employed in the known art to control elements of an optical phased array. The far-field technique has been used to align an array of semiconductor laser amplifiers by dithering the phases of the elements to achieve a beam with the narrowest far-field divergence. Unfortunately, the computational requirements of the near-field technique scale superlinearly in relation to the number of elements. Furthermore, the signal to noise ratio degrades linearly with the number of elements. As a result, laser systems comprising a large number of emitters for higher power output cannot be controlled real-time due to the computational time required.
Accordingly, there remains a need in the art for an optical phased array control system that comprise computationally efficient algorithms in order to provide real-time feedback to compensate for phase changes due to external perturbations. Further, the algorithms should preferably be capable of calculating phase measurements and corrections with relatively high fidelity. Moreover, the algorithms should be capable of generating arbitrary phase-fronts for corrections due to atmospheric aberrations.
In one preferred form, the present invention provides an optical phased array control system comprising a master oscillator laser that produces a beam, which is split into a reference beam and a plurality of array beamlets. Generally, the phase of the reference beam and the phase of the array beamlets are measured and compared, wherein a computationally efficient algorithm computes both phase measurements and phase corrections. Accordingly, phase correction signals are transmitted to a plurality of phase modulators that correspond to each of the array beamlets. The phase modulators then realign the phases of the array beamlets to compensate for external perturbations.
In operation, the array beamlets are first transmitted through a first fiber array where the beamlets are diverged into a first microlens array. Accordingly, the first microlens array generates a composite beam that is transmitted as an output beam from the system. The composite beam is further transmitted and interfered with a reference plane wave that is generated from the reference beam. The composite beam and the reference plane wave are then transmitted through a second microlens array where the array beamlets and the reference plane wave are focused to a second fiber array. The second fiber array then transmits the array beamlets and the reference plane wave to a plurality of photodetectors, which correspond with the array beamlets, wherein the photodetectors detect the phases of the reference plane wave and the array beamlets. Subsequently, the output from the photodetectors is transmitted to a computing device, such as a personal computer (PC), wherein the algorithm calculates a phase measurement and corrections.
The phase measurement is generally the difference between the phase of a specific array beamlet and the phase of the reference plane wave. In addition, the phase correction for each array beamlet is calculated based on the phase measurement, as described in greater detail below. The phase correction signal is then transmitted to the phase modulators, which realign the phases of the array beamlets to match the phase of the reference beam such that a flat phase front is generated. Alternately, an arbitrary phase-front is generated to correct for atmospheric aberrations.
As a part of the real-time control system, the reference plane wave is ramped from 0 to 2xcfx80 radians using a reference phase modulator. Preferably, the ramp is achieved in four equally spaced steps for optimal results. Accordingly, a trigger is employed to send signals to the phase modulator and to the personal computer, wherein the algorithm uses a signal from the trigger to calculate the phase measurements and corrections as described in greater detail below.
The phase measurement according to the algorithm is calculated based on a difference between the phase of the array beamlets and the phase of the reference plane wave. More specifically, the phase measurement is based on a phase angle of a complex number comprising square wave functions. The square wave functions are insensitive to AC amplitudes and DC components of the phase of the array beamlets and the phase of the reference plane wave, and as a result, the computations are less intensive and are capable of supporting real-time feedback to the phase modulators. In another form, the phase measurement is calculated based on a phase angle of a complex number comprising sine and cosine functions to correct for harmonic content of the phases of the array beamlets and the reference plane wave.
The phase corrections are calculated based on the phase measurement calculation times modulo2xcfx80. The phase correction signal is cumulative, and thus a current phase correction is subtracted from a previous phase correction for each cycle that measures phases and calculates the phase measurements and corrections. Accordingly, the cycle must be faster that the external perturbations in order to provide real-time phase control.
In another form, a plurality of PCs may be employed for a large number of array beamlets, thereby reducing the computing load. As a result, a parallel real-time phase control system may be used that is capable of controlling relatively large optical phased arrays without exceeding available computing resources.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.